The invention relates to a continuously variable transmission of the type employing pairs of conical disks and an endless flexible torque-transmitting device (chain-belt, for short).
A transmission of this kind is known, e.g., from DE 3 447 092 A1. In this transmission, the torque is transmitted from one pair of conical disks to the other by means of a chain of steel links with force-transfer elements arranged transverse to the lengthwise direction of the chain. The end surfaces of the force-transfer elements are in frictional engagement with the friction surfaces that face each other in each of the pairs of conical disks. Both the end surfaces of the force-transfer elements and the friction surfaces of the pairs of conical disks are of a convex, rounded shape. The convex curvature radii are selected in such a way that overall, the different portions of the end surfaces of the force-transfer elements that come into frictional contact with the friction surfaces at different transmission ratios are distributed essentially over the entire end surface, so that the end surface material is used optimally and the chain-belt transmission has a long work life.
It is in the nature of chain-belt transmissions, particularly those used in motor vehicles, that over the entire operating life, the amounts of time spent in different parts of the range of transmission ratios are unequal. For example, the chain-belt transmissions of vehicles that are not exclusively used in city traffic are operated at the highest ratio for an over-proportionate amount of the time, i.e., with the pair of disks on the input side of the transmission running at their closest distance and the pair of disks on the output side running at their widest distance, in order run the vehicle as economically as possible. Therein lies the danger that the driven pair of disks in particular could become worn, even leading to the formation of grooves. The formation of grooves or ruts in the friction surfaces of a pair of conical disks has the effect that very small changes of the distance between the disks lead to big changes of the transmission ratio. This results in instabilities in ratio-shifting and/or makes it impossible to down-shift, i.e., to move the chain-belt towards an area of greater radius on the rutted pair of conical disks.
Also, instabilities in the gripping-force ratio occur for example after a transmission has been in operation for a long time, which requires a corresponding compensation adjustment to be made by the control unit through the control or regulation of the compressive force and the axial displacement of the pairs of conical disks. The gripping-force ratio in this context is the ratio between the respective amounts of axial force at the two pairs of conical disks. Particularly critical are conditions where the gripping-force ratio has a steep gradient in relation to the transmission ratio. This can lead to problems with the control and regulation and to oscillations of the transmission ratio, if the gripping-force ratio changes over the operating life of the transmission. To keep the transmission ratio constant, it is advantageous if the gripping force ratio is stable within a given range.
The object of the present invention is to further develop a chain-belt transmission of the kind discussed above with the aim of improving its long-term functionality and assuring stable regulation and control properties to the greatest extent possible in all operating states.
A continuously variable chain-belt transmission meeting the aforementioned objective has an input shaft and an output shaft running parallel at a distance from each other. The input and output shaft each carry a pair of conical disks rotationally constrained on the respective shaft, while the axial width between the conical disks of each pair is variable so that an increase in the width of one pair is tied to a decrease in the other and vice versa. An endless flexible torque-transmitting device (which will subsequently be referred to as a chain belt) loops around the essentially cone-shaped friction surfaces of the conical disks. Transverse members (subsequently called force-transfer elements) arranged along the chain belt have end surfaces in frictional contact with the friction surfaces of the pairs of conical disks, thereby enabling the chain belt to transmit torque from the input shaft to the output shaft. The continuously variable transmission according to the invention is advantageously distinguished in that the friction surfaces of at least one of the pairs of conical disks have at least one diameter zone where the contour of the friction surface deviates from a uniform contour shape.
Preferred as a uniform contour shape (in a sectional view) is either a straight-line contour or a circular segment of a given radius.
It is practical if the deviating contour is distinguished by having a portion of greater or lesser material depth that could be formed by laying on or removing material.
It is further advantageous if the friction surfaces of at least the pair of disks on the output shaft have a convex curvature with respect to the radial direction, with a greater curvature radius R in the area of smaller radial distances from the shaft.
It is also of practical advantage if the friction surfaces have radially inner ring-shaped border zones of constant slope angle xcex11.
It is of benefit if the friction surfaces of at least the disk pair of the input shaft and/or of the output shaft have, in order of decreasing radius, a zone of constant slope angle continuing through a concave zone into a convex zone with regard to the depth of material.
It is also of practical benefit if the end surfaces of the force-transfer elements are convex-rounded towards the friction surfaces of the conical disks, with the radius of the convex curvature increasing from r1 to r2 with increasing radial distance from the shaft.
A further configuration of the continuously variable chain-belt transmission has
a) an input shaft and an output shaft running parallel at a distance from each other, the input and output shaft each carrying a pair of conical disks rotationally constrained on the respective shaft, while the axial width between the conical disks of each pair is variable so that an increase in the width of one pair is tied to a decrease in the other and vice versa;
b) a chain belt looping around the essentially cone-shaped friction surfaces of the conical disks, with transverse force-transfer elements arranged along the chain belt, the force-transfer elements having end surfaces in frictional contact with the friction surfaces of the pairs of conical disks, thereby enabling the chain belt to transmit torque from the input shaft to the output shaft;
and is distinguished by the advantageous feature that the friction surfaces of at least one of the pairs of conical disks have a contour which, as a function of radial distance from the shaft, deviates from a straight-line or a circular segment-shaped contour.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both in its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.